Question

Which of the following belongs to the group of Doberiener's triad?

• A. Calcium, strontium, and barium
• B. Chlorine, bromine, and iodine
• C. Hydrogen, lithium, and sodium
• D. Phosphorus, vanadium and niobium

Answer 1

Answer: (A), (B)

Chlorine, bromine, and iodine

1) Atomic masses of $\text{Ca}$ = 40, $\text{Ba}$ = 137,
$\text{Sr}$ = 88
We know that:
${\text{Atomic wt. of 2}}^{\text{nd}}$ element= $\frac{{\text{Atomic wt. of 1}}^{\text{st}}+{\text{Atomic wt. of 3}}^{\text{rd}}}{2}$
Here, $\frac{\text{Atomic wt. of Ca + Atomic wt. of Ba}}{2}$
= $\frac{137+40}{2}$ =88.5 ~ 88 = the atomic mass of $\text{Sr}$
Hence, it is Dobereiner's triad.

2) Atomic masses of $\text{Cl}$ =35, $\text{I}$ =127, Atomic mass of $\text{Br}$ =80
$\frac{\text{Atomic wt. of Cl + Atomic wt. of I}}{2}$
= $\frac{35+127}{2}$ = 81= the atomic mass of $\text{Br}$
Hence, it is a Dobereiner's triad.

3) Atomic masses of $\text{H}$ =1 , $\text{Li}$ =6, $\text{Na}$ =22
$\frac{\text{Atomic wt. of H + Atomic wt of Li}}{2}\ne \text{Atomic wt. of Na}$
Hence, it is not a Dobereiner's triad.

4) Atomic masses of $\text{P}$ =30, $\text{V}$ =50, $\text{Nb}$ =92
$\frac{\text{Atomic wt. of P+ Atomic wt. of Nb}}{2}\ne \text{Atomic wt. of V}$
Hence, it is not a Dobereiner's triad.